With regard to many electronic and electrical systems, such as computer and home entertainment systems, electrical power must be applied to the components of the system according to a particular sequence to avoid causing undue stress and possible damage to them. For example, in many computer systems, power must first be applied to the computer before being applied to the monitor, because the computer supplies the monitor with horizontal and vertical synchronization pulses necessary to prevent the free running of the monitor's horizontal and vertical oscillators. Allowing the oscillators to operate in an unsynchronized condition can result in undue stress to the oscillators and hard failure of the monitor.
Similarly, power must usually be applied to the computer before being applied to the printer. Otherwise, the printer might apply power to the computer and potentially cause the computer to fail to initialize when power is subsequently applied to it. Consequently, the order and timing of the application of power to and removal of power from certain systems must be carefully controlled so as to avoid damaging the system components. It is often essential to proper power up and operation of an electronic or electrical system that power be applied to a controller component, such as the computer, before being applied to subordinate components, such as the monitor or the printer. For the same reasons, it will usually be desirable for the components to be turned off in the reverse order.
One solution is for the system user to turn on the components manually, allowing sufficient time after power up of the controller component to ensure that it has reached a stable state before applying power to the subordinate components. This method is generally unsatisfactory, because the time delay interval is difficult to control and duplicate manually, and further, because it may be desirable to ensure that the power up and power down of the system always occur according to a particular sequence.
Another solution is to use time delay relays (TDRs) to provide a predetermined, fixed time delay between application of power to one component and the next. This method is also unsatisfactory, as well as being very expensive. TDRs are capable only of providing a fixed, or at best, a narrowly adjustable, time delay. Furthermore, the power up delay is exactly equal to the power down delay, a condition which may be undesirable in certain cases. Finally, TDRs have no memory means in which a preferred power up sequence can be stored; therefore, the sequence cannot easily be changed by a user.